On Sun, 2011-05-29 at 08:58 +0200, Pavel Machek wrote:> On Fri 2011-05-27 13:45:51, David Safford wrote:> > On Thu, 2011-05-26 at 22:17 +0200, Pavel Machek wrote:> > > > > I suggest you explain the patchset in the emails, then? Everyone here> > > seems to be confused... Attack it protects against, and what kind of> > > hardware is needed for the protection to be effective?> > > > The white paper is over 15 pages, and it barely scratches the surface.> > Every customer has different security threat models and requirements.> > Discussing this in general on the mailing list is really hard.> > > > So let's try to simplify this just down to digital signatures in> > the cellphone environment, as you state:> > Good.> > > > Because AFAICT, file signatures, as proposed, are only useful for> > > locking down my cellphone against myself. (That's -- evil).> > > > The proposed digital signatures can enforce authenticity of a file's > > data (IMA-Appraisal with Digital Signature), and of a file's metadata> > (EVM with Digital Signature). For most users, enforcing authenticity> > of files is a good thing - a user knows that they are running authentic> > software signed by their phone manufacturer, and not malicious files> > that they, or someone else installed. In this threat model, EVM is > > Ok, so lets talk about smartphone, similar to my HTC Dream (developer> version, unlocked bootloader, flashable from kernel (*)).> > Yes, I could install the crazy EVM/IMA infastructure to prevent> applications modifying selected files.> > But... I could just do chattr +i on selected files, I do not need> fancy EVM/IMA for that.

For files that you don't expect to change, such as ELF executables, youprobably could use the immutable flag, but using a digital signatureprovides authenticity as well, which the immutable flag does notprovide.

> > Blocking signature verification would serve only to punish Linux > > users who care about the authenticity of their files, while doing > > _nothing_ to stop manufacturers from locking their bootloaders.> > chattr already protects authenticity of my files, as do standard unix> permissions.> > So... where's the difference?> Pavel

Neither digital signatures nor the immutable flag work for files thatchange, such as config files. For these files, ima-appraisal wouldstore a file hash.

> (*) but it does not change anything.> > True; determined attacker could steal my cellphone, open it up,> desolder the flash, and change attributes of the filesystem.

With EVM, assuming that i_flag is included in the EVM HMAC, which itcurrently isn't, you would be able to detect the change and prevent thefile from being accessed.

> > But... the same determined attacker can also replace> bootloader&kernel&filesystem -- that is in the same flash! -- with> unlocked versions. So the argumentation is the same for locked down> phone.>

As EVM is not involved in the boot process, it can not and does notaddress this, but other technologies could.